Method and apparatus for cardiologic echo-doppler image enhancement by gated adaptive filtering in time domain

ABSTRACT

A method of enhancing the image quality of cardiologic and general untrasound echo-doppler apparatus by finite memory, ECG-gated and/or breathing gated filtering of the acquired image. The acquired scans are time-gated by the cardiac contraction cycle, or by the breathing cycle to account for cardiac deformation due to contraction or relaxation, and to cardiac movements due to lung inflation or deflation, or to cardiac-induced pulse of perfusion in non-cardiac organs and tissues. The gated scans are recursively filtered with saved images, which are processed scans of previous cardiac and/or breathing cycles. The resulting images are displayed and also saved for the next cycle filtering process.

This appln. is a 371 of PCT/US97/09455 filed May 28, 1997, and alsoclaims benefit of Provisional No. 60/018,466 filed May 28, 1996.

BACKGROUND OF THE INVENTION

Echo-doppler is commonly applied for non-invasive imaging of the heart.Commercially available systems provide the user with several imagingmodalities: 2D two-dimensional) image of a single plane through theheart; m-mode image of a single line through the heart; doppler image ofa flow in a specific location (pulsed doppler) or in any location alonga line (continuous doppler); color-doppler image which consists ofsuperimposing information of flow direction and velocity on 2D image;and tissue-doppler imaging which provides tissue deformation data on a2D image. Three-dimensional imaging of the heart, which currentlyprovides off-line reconstruction of the heart from sequentially acquiredmulti-plans of the heart may soon provide real-time imaging of theheart.

A main limitation in the clinical application of echo-doppler imaging isthe image quality. While about 20-30% of the studies have good technicalquality and do not require image enhancement, about 40-60% areacceptable studies which can be enhanced to get more accurateevaluation, and about 20-30% are technically poor and usually do notprovide the required image quality.

This limitation is either overcome by using oesophageal imaging, ratherthan the standard trans-thoracic imaging, or by using echo-contrastmedia which are injected intravenously. These techniques significantlyimprove the image quality but are semi-invasive and can only be used ina limited number of studies.

Accordingly, there exists a need for a system that will provide forimage enhancement of trans-thoracic studies that can significantlyimprove the accuracy of echo-doppler imaging in the majority of studieswhich provide images that can be analyzed but need more time to optimizethe image and to get the required clinical data.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand apparatus for image enhancement in cardiologic applications ofecho-doppler imaging.

It is another object of the present invention to improve the imagequality of the technically poor studies sufficiently to get the requireddata and to avoid the use of the semi-invasive trans-oesophageal andecho-contrast techniques.

Yet another object of the present invention is to enhance perfusionstudies of the heart or other organs which are currently done withcontrast-enhanced echo-doppler imaging.

According to the present invention, repetitive physiologic phenomena—theheart contraction/relaxation cycle and/or the breathinginspiration/expiration cycle, are divided into “m” equal sequences.Factors that determine “m” include the length of the physiologic cycle,the scanning rate of the echo-doppler apparatus, the image area to beenhanced and the amount of available memory in the processing apparatus.Typically, “m” equals 20-40. The image acquired during each sequencerepresents the heart shape at the specific time of the measurement withrespect to a fixed reference point in the physiologic cycle (e.g. theinitiation of heart contraction, signaled by the R-wave of the ECG, orthe initiation of the breathing cycle, signaled by the initial portionof inspiration). This image is to be filtered with previous images atthe same time, relative to the reference point, that were acquired inprevious physiologic cycles. Thus the change of the heart shape due tothese physiologic phenomena is eliminated and the images can be filteredto get an improved image.

The improved image is displayed on the monitor (either with the acquiredraw image or as substitute) and is saved for filtering with futureimages. A simple embodiment of this filtering is weighted averaging,where the newly acquired image is multiplied by a certain factor, theimage saved during previous cycles is multiplied by another factor andthe two results are summed. The multiplying factors (the multipliers)determine the “memory” of the algorithm, or the effect of previousimages versus the effect of the newly acquired image, and can becontrolled by the operator.

More complex filtering requires more memory which can be used to saveseveral weighted past images, and thus the newly acquired image can befiltered with several previous images. This approach can be applied withall modes of commercial echo-doppler systems, namely 2D, m-mode,doppler, color-doppler, tissue-doppler and 3D imaging.

The same approach can be used for perfusion studies of any organ in thebody. Due to the pulsatile pattern of blood flow and pressure in thebody, gating to the R-wave of the ECG enhances the quality of theperfusion images and may allow quantitated analysis of perfusionpulsatility in any organ. This may assist the delineation of abnormaltissue, for example a tumor, from neighboring normal tissue.

The invention also provides apparatus to enable the application of thedescribed method either as an add-on device for commercial echo-dopplersystems or as a built-in module for future echo-doppler systems. Theapparatus comprises an interface to acquire the image from theecho-doppler system (e.g. video frame grabber), an interface to get thephysiologic cycling signal (e.g. the ECG raw signals which can beprocessed to provide both heart contraction timing and breathing timingor the ECG tracing which is commonly recorded as part of theecho-doppler display), memory modules to save the gated images, userinterface to control parameters of the algorithm (e.g. the multipliersfor weighted averaging), micro-processor for mathematical processing ofthe imaging (e.g. array processor), and an interface to present theenhanced image (e.g. an additional screen or interface to theecho-doppler screen).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the proposed system according to thepresent invention; and

FIG. 2 describes the physiologic cycling phenomena—the heart contractioncycle and the breathing cycle, and the implementation of the algorithm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is illustrated an ultrasonic echo-dopplerscanner 12 which provides raw scanned images 14 of the heart that areprovided to a processing unit 16, where they are stored. A physiologicsignal 18, such as the R-wave of the ECG, is provided by physiologicmonitor 20 to the processing unit 16. Physiologic monitor may be anexternal device, such as an ECG monitor, or a software module for theextraction of the ECG tracing from the echo-doppler display.

Signal 18 provides a timing-gating signal through which the ultrasonicimage 14 is received by processing unit 16, i.e., each image is taggedwith a specific time in cycle with reference to a physiologicalreference point. A total of “m” tagged images are created if thephysiological cycle is divided into “m” sequences.

Various user controls, employing the parameters of an image enhancementalgorithm, are set by the user through a user interface unit 22. Thescanned raw images 14 are processed in real-time to enhance the imagequality to provide an enhanced image 24 which is provided to display 26for viewing. The “m” processed images are displayed on display 26 withtheir corresponding times in reference to the physiological referencepoint.

The image enhancement algorithm is schematically presented in FIG. 2.The acquired image 14 at a specific time in reference to a referencepoint of the physiologic signal 18 (e.g. the R-wave of the ECG) isdenoted as image k. Taking each pixel (i,j) of the image k, denoted byP(k,i,j), the new value for the pixel, denoted by P′(k,i,j), can becalculated as being a combination of a function F1 of the current pixelvalue and a function F2 of the pixel value in n previous images (denotedby P′(k-l . . . k-n,i,j)). These images are from previous physiologiccycles taken at the same time with reference to the physiologicalreference point. The function F1 is determined by a set of parameterscontained in a vector v1(k), and the function F2 is determined byanother set of parameters contained in a vector v2(k). The parameters inthe vectors v1(k) and v2(k) can be either fixed values or changingaccording to the nature of the image (adaptive parameters).

The image enhancement algorithm can be written according to thefollowing formula:

P′(k,i,j)=F1[P(k,i,j),v1(k)]+F2[P′(k-l, . . . k-n,i,j),v2(k )]

The processed image consisting of all the processed pixels P′(k,i,j) isdisplayed to the user as an enhanced image on display 26 which is eitherpresented in addition to the raw image 14 or substituted for the latter.The same process is applied individually to all “m” images that comprisea complete physiological cycle, so that an enhanced, real-time dynamicimage results.

The same approach can be used for enhancement of perfusion imaging ofthe heart or any other organ or tissue in the body. Although mostnon-cardiac organs do not move with the heart cycle, the blood flow andblood pressure varies with time, synchronized with heart contraction.Perfusion studies are performed by injecting echo-contrast material intothe blood which enhances the contrast between blood and other tissuesand thus demonstrate different levels of blood perfusion into the sametissue. Gating the acquired images to the R-wave of the ECG enables theenhancement, through averaging of the image without losing the variationof image intensity due to pulsatility-induced changes in blood volume inthe scanned tissue.

An equivalent approach is used with m-mode and doppler imaging, but theenhancement is applied on columns of pixels rather than the whole 2Dimage. Consequently, each new column of pixels is filtered with nprevious columns which were acquired at the same time in reference tothe reference point of the physiologic signal.

The user can control the parameters of the vectors v1 and v2 throughuser interface 22. Possible controls may include the use of adjustableslide bars (not shown) that can be moved to determine the required levelof each parameter or through a software interface by using a mouse orother keyboard buttons to change values on a computer screen.

The following is a description of one manner of using the system, basedon weighted averaging of the image, to enhance the acquired image duringa routine echo study.

The user positions the sensing portion of the ultrasonic scanner 12 in acertain “echo window” which provides the required image of the heart.Initially, a slide bar which determines the level of the parameter v2which is the weighing factor for previous images is set to zero level,so the processed image is identical to the raw image. Once the user issatisfied with the position of the transducer and the acquired image,the user slides the bar to gradually increase the level of the parameterv2. The larger the parameter is, the larger the effect of the priorimages on the processed image (the image memory is longer). Obviously,the longer the period of time where the transducer is held in a fixedposition and the scanned image does not move, more images will beaveraged and the image quality will improve. However, since time is alimiting factor in clinical studies and since as the memory becomeslonger the effect of additional images become smaller, there is anoptimal level for the parameters v1, v2, which may result in optimalimage within a reasonable time. Although it will be subjected to theexperience and preferences of the user, a reasonable image enhancementof the contracting heart can be obtained by recursively averaging 10-25heart cycles when the scanned subject is asked to hold breathing. Thistheoretically may result in improvement of the signal to noise ratio bya factor of 2-5, depending on the level of the parameters v1, v2. If thegating is on both the heart cycle and breathing cycle, longer scans canbe averaged until a satisfactory image is obtained.

For perfusion studies, after injection of the contrast media andachieving the required image of the organ, the operator will typicallyset the parameters of v1, v2 to a long memory setup and continue asteady state imaging for the required time to achieve the required levelof image enhancement. Longer periods of up to several minutes mayachieve the best results by eliminating the effect of breathing on organposition and blood flow fluctuation.

It will thus be seen that the illustrated apparatus and method can beused for the enhancement of the image quality of routine echo-dopplerimaging of the heart. The present invention may improve the resultingimages in most studies performed so that more accurate measurement andanalysis can be done. In some cases it may provide sufficient imagequality to avoid the use of semi-invasive methodologies which arecurrently required when the trans-thoracic image is technically poor. Inother cases, the apparatus and method can be used with contrast echo toenhance the quality and expand clinical applications of blood perfusionstudies. The apparatus is simple to use and the operation of the methoddoes not require significantly more time than is currently required toget a high quality image for measurements and analysis. The inventioncan be also applied during exercise echo-cardiography, where the imagequality is usually limited and complicates the real-time monitoring andthe off-line analysis of the study.

While a particular system has been described above in conjunction withFIGS. 1 and 2 for generating an enhanced echo-doppler image, theinvention is not limited to the specific system disclosed and othersystems performing these functions are within the contemplation of theinvention. Thus, while the invention has been particularly shown anddescribed above with reference to a preferred embodiment, the foregoingand other changes in form and detail may be made therein by one skilledin the art while still remaining within the spirit and scope of theinvention.

What is claimed is:
 1. An echo-doppler image enhancement system,comprising: a processing unit for receiving and storing scannedultrasonic images, physiologic signals and a filtering input; ultrasonicmeans for providing scanned ultrasonic images of an organ of a subjectof said unit; a physiologic sensor for providing said physiologic signalof the subject for gatining said scanned ultrasonic images so that saidimages are tagged by a specific time within a physiologic cycle withreference to a physiologic reference point when they are received bysaid unit; means coupled to said processing unit for providing saidfiltering input for modifying said gated ultrasonic image according to afilter applied on a set of said images from different physiologic cyclesat the same specific time within a physiologic cycle to enhance theultrasonic image; and display means coupled to said processing unit forreceiving and displaying said enhanced ultrasonic image.
 2. An echoenhancement system according to claim 1, wherein said organ is the heartand said physiologic cycle is the cardiac cycle.
 3. An echo enhancementsystem according to claim 1 wherein said organ is any organ of the bodywhich is studied with ultrasonic contrast media.
 4. An echo enhancementsystem according to claim 1, wherein said filtering input operatesaccording to the following formula:P′(k,i,j)=F1[P(k,i,j),v1(k)]+F2[F2[P′(k-l, . . . k-n,i,j),v2(k)] whereinP′ is the enhanced image, P is the raw scanned image, k indexes thephysiologic cycle, i and j index the pixel location, v1 and v2 are userdefined vectors defining the nature of the applied filtering of theimage.
 5. An echo enhancement system according to claim 1, wherein saidphysiologic cycle is the breathing cycle.
 6. A method of enhancing anecho-Doppler image comprising the steps of: detecting an ultrasonicimage of a structure to be displayed; gating said ultrasonic image witha physiologic signal so that each image is tagged with a specific timewithin a physiologic cycle with reference to a physiologic referencepoint; recursively filtering a set of said gated images from differentphysiologic cycles at the same specific time with the physiologic cycleby combining a weighted real time image with previously weighted imagesto create an enhanced image said gating and said filtering beingperformed on “m ” different images that are acquired during a completecycle of said physiologic signal; and displaying said enhanced “m ” intheir tagged times with reference to said physiologic reference point.7. A method of enhancing an echo-Doppler image according to claim 6,wherein said physiologic signal is an electrocardiogram of the heart andsaid physiologic cycle is a cardiac cycle.
 8. A method of enhancing anaccording to claim 6, wherein said step of recursively filtering saidgated image operates according to the following formula:P′(k,i,j)=F1[P(k,i,j),v1(k)]+F2[F2[P′(k-l, . . . k-n,i,j),v2(k)] whereinP′ is the enhanced image, P is the raw scanned image, k indexes thephysiologic cycle, i and j index the pixel location, v1 and v2 re userdefined vectors defining the nature of the applied filtering of theimage.
 9. A method of enhancing an echo doppler image according to claim6 wherein said image is an image of any organ or tissue in the body thatis studied with ultrasonic contrast media.
 10. A method of enhancing anecho-Doppler image according to claim 6, wherein said physiologic signalis a pneumogram and said physiologic cycle is the breathing cycle.